Hardware necessary to facilitate electronic signal intelligence gathering activity often includes a digital wideband receiver to detect multiple signals of unknown characteristics with a reasonable instantaneous dynamic range. In an effort to double the available bandwidth of a given system, other technology disciplines have implemented an in-phase/quadrature phase (I/Q) technique to extend the available sampling range beyond the primary Nyquist zone. Through this method, incoming signals are processed both in their native format, as well as in a 90° phase shifted format. By converting an input signal into both real and complex data, the available signal set may be extended to the conjugate Nyquist zone (from negative half of the sampling frequency to DC), thus doubling the bandwidth.
While the extended bandwidth realized by the I/Q technique is beneficial, operational problems have thus far prevented implementation in electronic signal intelligence gathering hardware. Unfortunately, variations in manufacturing tolerances in the quadrature phase shift hardware result in unavoidable imbalance in the I/Q channels. This produces an image signal, or alias signal (image and alias may be used interchangeably herein), in Fast Fourier Transform (FFT) analysis. As a consequence, the instantaneous dynamic range deteriorates.
Many of the techniques used to mitigate the effects of I/Q imbalance have been explored in signal processing disciplines wherein received signals are of a known frequency having well defined waveforms. The operational bandwidth is often narrow. As a result, many prior art techniques assume that the imbalance is frequency independent (since they are operating in a narrow band environment). However, electronic signal intelligence applications often include unknown frequencies or waveforms, and prior art imbalance compensation techniques are ineffective when imbalance correction is assumed to be frequency independent.
Further, prior art techniques that address frequency dependent mismatch issues, are incapable of applying mismatch correction when signals are simultaneously received in both Nyquist zones. In electronic signal intelligence scenarios, correction techniques must be capable of supporting simultaneous reception of signals in both Nyquist regions.
Therefore, there exists a need in the art for methods and apparatus to compensate for I/Q imbalances in wideband signal processing applications.